This invention relates generally to magnetic resonance imaging (MRI).
Magnetic resonance imaging (MRI) is a non-destructive method for the analysis of materials, and provides medical imaging. It is generally non-invasive and does not involve ionizing radiation. In very general terms, nuclear magnetic moments are excited at specific spin precession frequencies which are proportional to the local magnetic field. The radio-frequency signals resulting from the precession of these spins are received using pickup coils. By manipulating the magnetic fields, an array of signals is provided representing different regions of the volume. These are combined to produce a volumetric image of the nuclear spin density of the body.
MRI is based on nuclear spins, which can be viewed as vectors in a three-dimensional space. During an MRI process, each nuclear spin responds to four different effects: precession about the main magnetic field, nutation about an axis perpendicular to the main field, and both transverse and longitudinal relaxation. In steady-state MRI processes, a combination of these effects occurs periodically.
Compared with other modalities, such as X-ray, CT and ultrasound, MRI takes longer time, sometimes several minutes, for data acquisition to generate clinically useful images. Undesirable imaging artifacts may appear due to the long scan time.
Three dimensional (3D) MRI acquisition has the benefits including high available signal-to-noise ratio (SNR), full-volume coverage, and arbitrary image reformats. However, due to the relatively long scan time, the clinical application of 3D data acquisition was limited. With the development of parallel imaging (PI) and compressed sensing (CS) techniques, MRI data acquisition was significantly accelerated, which made 3D acquisition clinically feasible. PI uses a phased array with multiple receiving coils to simultaneously acquire data. The spatially varying coil sensitivities of the phased array are used to partially replace the traditional k-space encoding, therefore scan time is reduced. CS randomly undersamples data acquisition in k-space, and achieves image reconstruction by solving a nonlinear optimization problem that exploits the transform sparsity of the image. Combining PI and CS, higher acceleration in data acquisition can be achieved. As scan time is greatly reduced, the reconstruction computation by PI, CS or the combination of the two becomes significant, especially for 3D datasets.
U.S. Pat. No. 6,841,998 by Griswold, issued Jan. 11, 2005 entitled “Magnetic Resonance Imaging Method And Apparatus Employing Partial And Parallel Acquisition, Wherein Each Coil Produces A Complete K-Space Datasheet,” which is incorporated herein by reference for all purposes, also describes a GRAPPA based reconstruction. U.S. Pat. No. 7,692,425 by Brau, issued Apr. 6, 2010, entitled “Method and apparatus of multi-coil MR imaging with hybrid space calibration,” which is incorporated herein by reference for all purposes, discloses a parallel imaging system and Autocalibrating Reconstruction for Cartesian Sampling (ARC), which uses compressed sensing techniques to reconstruct an MR image.